Current developments of artificial small diameter vascular grafts for patients with cardiovascular disease involves both material-based (Deutsch et al., 1999 Surgery 126: 847-55; Gulbins et al., 2006 Thorac Cardiovasc Surg 54: 102-7; Heydarkhan-Hagvall et al., 2006 Tissue Eng 12: 831-42; Ma et al., 2005 Tissue Eng 11: 1149-58; Matsuda et al., 2005 J Biomed Mat Res 73: 125-31; Williamson et al., 2006 Biomaterials 27: 3608-16; Zarge et al., 1997 J Surg Res 67: 4-8) and cell-based (Niklason et al., 1999 Science 284: 489-93; L'Heureux et al., 1998 FASEB 12: 47-56) approaches. The advantage of off-the-shelf avail able, ready-made grafts from synthetic polymers would favor a material-based approach, although the re-occurrence of thrombus formation and the lack of healing impede the long-term patency of such grafts, especially for those with small diameter (<4 mm) (Zilla et al., 2007 Biomaterials 28: 5009-27). These problems are often due to the mismatch in mechanical properties (Matsuda et al., 2005, J Biomed Mat Res 73: 125-31; Sarkar et al., 2006 Eur J Vasc Endovasc Surg. 31: 627-36) between synthetic grafts and native vascular vessels, as well as the absence of endothelial ingrowth in the midgraft area even after 10 years post surgery (Zilla et al., 2007 Biomaterials 28: 5009-27). Therefore, the fabrication of artificial grafts that possess elastic properties similar to those of native arteries and capable of facilitating endothelialization can significantly improve the healing of small-diameter grafts.
Current techniques of constructing 3-dimensional (3-D) synthetic vascular grafts include suturing a 2-D polymeric sheet into a 3-D tubular structure (Heydarkhan-Hagvall et al., 2006 Tissue Eng 12: 831-42) and electrospinning of polymer solution onto a rotating mandrel (Matsuda et al., 2005 J Biomed Mat Res 73: 125-31; Williamson et al., 2006 Biomaterials 27: 3608-16; Courtney et al., 2006 Biomaterials 27: 3631-8). Utilizing the electrospinning technique, various luminal surface topographies such as pores, non-woven mesh and oriented fibers can be created by adjusting the mandrel speed. Depending on the viscosity of polymer solution, electrospun fibers vary from nano- to micro-scale in diameter. In terms of mechanical property, the mesh morphology consisting of physically entangled fibers provide the mesh-type vascular scaffolds excellent compliance, compared to other structures, to withstand the pulsatile arterial flow (Matsuda et al., 2005 J Biomed Mat Res 73: 125-31).
Both porous (Zilla et al., 2007 Biomaterials 28: 5009-27) and mesh (Ma et al., 2005 Tissue Eng 11: 1149-58) topographies have been shown to promote capillary ingrowth, spreading, adhesion, mid proliferation of endothelial cells. Oriented-fiber topography guides endothelial cell alignment, similar to the groove-like topography that induces the alignment of many cell types (Oakley et al., 1995 Cell Motil Cytoskeleton 31: 45-58; Jiang et al., 2002 Langmuir 18: 3273-3280; den Braber et al., 1996 Biomaterials 17; 2037-44; den Braber et al., 1998 J Biomed Mater Res 40: 291-300; Uttayarat et al., 2005 J Biomed Mater Res 75: 668-80). This alignment of endothelial monolayer emulates the naturally aligned and elongated endothelium in linear vascular vessels under hemodynamic flow environment (Uttayarat et al., 2005 J Biomed Mater Res 75: 668-80; Nerem et al., 1981 J Biomech Eng 103: 172-6) and also guides directional cell migration (Uttayarat et al., 2008, Am J Physiol Heart Circ Physiol. 294:1-H1027-35). A recent fabrication of synthetic graft combines both mesh and oriented fiber morphology as demonstrated in the polycaprolactone-polyurethane (PCL-PU) composite vascular graft (Williamson et al., 2006 Biomaterials 27: 3608-16), where the lumen exhibits oriented PCL microfibers and the exterior comprises highly porous PU (about 10 to 30 μm in diameter).
Electrospinning was developed in the textile industry in the 1930s (Bergshoef et al., 1999 Adv Mater 11: 1362-1365; Huang et al., 2003 Composites Sci Technol 63: 2223-2253; Jin et al. 2002 Biomacromolecules 3: 1233-1239) and has recently been applied to tissue engineering as a versatile platform technology for generating biomimetic fibrous scaffolds to be used as grafts such as synthetic vascular vessels (Drasler et al., 1993 ASAIO J 39: 114-119; Ma et al., 2005 Tissue Eng 11: 1149-58; Matsuda et al., 2005 J Biomed Mat Res 73: 125-31; Vaz et al., 2005 Acta Biomater 1: 575-582; Williamson et al., 2006 Biomaterials 27: 3608-16), cardiac patches (Hidalgo-Bastida et al., 2007 Acta Biomater 3: 457-462; Stankus et al., 2006 Biomaterials 27:735-744) and wound dressing (Chong et al., 2007 Acta Biomater 3: 32) -330; Khil et al., 2003 J Biomed Mater Res B Appl Biomater 67: 675-679; Rho et al., 2006 Biomaterials 27: 1452-61; Zhou et al., 2008 Biomacromolecules 9: 349-354). During the electrospinning process, the fiber size, fiber density as well as fiber organization can be modulated, thus enabling tight control over the scaffold structure. This complexity of the electrospun scaffolds emulates several features of natural extracellular matrix (ECM), which possesses pores and topographic cues for cell adhesion and proliferation (Li et al., 2006 J Biomed Mater Res A, 79: 963-73; Ma et al., 2005 Tissue Eng 11: 1149-58). Previous studies on synthetic vascular vessels made from Dacron or Goretex (Zilla et al, 2007 Biomaterials 28: 5009-27) have shown transmural ingrowth of capillaries through micro-scale pores in the vessel walls thus allowing the migration of vascular endothelial cells to cover the synthetic surface, in addition, for gratis embedded with directionally oriented microfibers, endothelial cells aligned their cell shape parallel to the fiber direction (Ma et al., 2005 Tissue Eng 11: 1149-58), similar to the flow-induced cell alignment observed in vivo (Nerem et al., 1981 J Biomech Eng 103: 172-6).
Polyurethane (PU) is preferred over traditional materials, such as Dacron and Cortex, for the fabrication of cardiovascular prostheses (Sarkar et al., 2006 Eur J Vasc Endovasc Surg. 31: 627-36; Tiwari et al., 2002 Cardiovasc Surg. 10; 191-7). Recently, polyurethane has been incorporated in the electrospinning process to fabricate synthetic vascular vessels (Ma et al., 2005 Tissue Eng 11: 1149-58; Matsuda et al., 2005 J Biomed Mat Res 73: 125-31; Williamson et al., 2006 Biomaterials 27: 3608-16) as well as heart leaflets (Courtney et al., 2006 Biomaterials 27: 3631-3638). For example, Courtney et al showed that poly(ester urethane) ureas electrospun onto a rotating mandrel exhibited anisotropic compliance, in which the scaffold is stiffer when stretched in the direction of aligned fibers compared to when stretch in the orthogonal direction to fibers. This anisotropic mechanical property strongly resembles the native pulmonary valve leaflet, as it requires almost 150% more stretch in the radial direction than in the circumferential direction to achieve the same membrane tension (Courtney et al., 2006 Biomaterials 27: 3631-3638). Concomitantly, the innate mechanical property of PU, the organization of electrospun PU fibers endows the scaffold with anisotropic compliance to closely mimic some of the more intricate mechanical properties of natural vascular tissues.
Angiogenesis is the formation of new blood vessels from established vascular beds. This complex process involves the migration and proliferation of existing vascular endothelial cells (EC), the formation of immature EC tubules, and maturation stages in which mesenchymal cells are recruited and differentiate into the pericytes or smooth muscle cells of the outer vessel layers (Risau, 1997 Nature 386: 671-674; Hanahan, 1997 Science 277: 48-50; Jain et al., 1997 Nature Medicine 3: 1203-1208).
Endothelial cells promote healing of damaged blood vessels within the body by promoting angiogenesis. In addition, endothelial cells can inhibit platelet adhesion and thrombus formation on blood-contacting surfaces.
Despite advances in tissue engineering, current three-dimensional vascular grafts generally lack structure sufficient to achieve adequate cell attachment and endothelialization to recapitulate natural endothelium. Accordingly, there remains a need for three-dimensional vascular grafts having suitable properties such as being able to support endothelial cell attachment, endothelial cell alignment, cell proliferation, and maintaining endothelial cell function in vivo. The invention fulfills this need.